1. Field of the Invention
The present invention relates to a process for the preparation of maleic anhydride, and more particularly to a process for preventing the formation of scales in apparatus during concentration and dehydration operations of an aqueous solution of maleic acid.
2. Description of the Prior Arts
Maleic anhydride has been generally prepared by concentration and dehydration of an aqueous solution of maleic acid, which is obtained by absorbing in water, in an apparatus such as a scrubber, a gas containing maleic anhydride, formed by catalytic oxidation of benzene, or gaseous by-products containing maleic anhydride, formed during the production of phthalic anhydride by catalytic oxidation of o-xylene or naphthalene.
The concentration and dehydration operations described above are performed by concentration and dehydration of an aqueous solution of maleic acid within a short period of time in apparatus such as a rotary film evaporator and a falling film evaporator, the water contained in the charged aqueous solution and the water formed by the dehydration reaction are evaporated, and maleic anhydride which is formed in the liquid phase is recovered in a molten state, or maleic anhydride is evaporated with the said water and the vapor mixture containing water and maleic anhydride is cooled to a temperature which does not allow water to condense and only maleic anhydride is separated and recovered in pure form (Japanese Patent Disclosure Nos. 80816/1976 and 50316/1975).
A variety of by-products are formed by the above-described processes for the preparation of maleic anhydride, and when an aqueous solution of maleic acid containing such by-products is processed as described above, there takes place accumulation of high-boiling point products such as resinous, tarry or gelatinous substances on the inner walls of the concentration-dehydration apparatus or pipes, and they tend to clog the apparatus and pipes.
In order to overcome these deficiencies, Japanese Patent Disclosure No. 80816/1976 discloses a process for the preparation of maleic anhydride in which the molten maleic anhydride obtained and discharged from a rotary film evaporator is circulated into a funnel-shaped discharging section provided at the lower part of the evaporator, through a nozzle which is positioned at about tangential direction to the wall of the discharging section. Japanese Patent Disclosure No. 50316/1975 (corresponds to British Patent Application No. 40528/1973) adopts a process in which the concentrated liquid is taken out from a distillation-dehydration PG,4 vessel, the concentrated liquid is filtered to remove the insoluble resinous residue, and the filtrate is circulated to the first scrubber of the process.